1. Field of the Invention
The present invention relates to an imaging lens and an imaging apparatus, more particularly, to an imaging lens suitable to be used in a surveillance camera, a cell phone camera, an onboard camera, and the like employing an imaging device such as a CCD (Charge Coupled Device) and a CMOS (Complementary Metal Oxide Semiconductor), and an imaging apparatus having the imaging lens.
2. Description of Related Art
Recently, systems, which have imaging apparatuses such as cameras mounted on a vehicle, for recognizing external environment by taking images of the front, the side, the rear, and the like of a vehicle, that is, for example, systems for sensing a line painted on a road (including a white line, a yellow line, and the like) or detecting obstacles have been practically used. In onboard lenses applied to such onboard cameras, it is important that the onboard lens is made to be a fast optical system and has a good imaging performance. Also, in the onboard lenses, it is important that a small part of the lens is exposed to the outside in consideration of an exterior view of a vehicle.
However, in such onboard lenses, by reflection of light such as headlight of the oncoming vehicle on a lens surface at the time of using an onboard lens, sometimes ghost image is formed. Sometimes the formed ghost image may reduce recognizability with respect to external environment depending on its level.
Particularly, in onboard cameras using solid-state imaging devices such as CCD, an image is displayed on an imaging plane by employing an auto gain function for automatically setting brightness sensitivity, and thus the following arises. A ghost image formed on an imaging plane by multiple reflections is darker than a headlight image formed on the imaging plane by headlight of the oncoming vehicle that is transmitted through a lens system without reflection. Even when the ghost image is formed in a state where the headlight image of the oncoming vehicle remains in the imaging plane, the auto gain function sets a gain based on bright light of the headlight. Thus, a low gain is set, and so the ghost image does not very stand out.
However, even after the oncoming vehicle passed and the headlight image disappeared, sometimes the ghost image remains in the imaging plane because of multiple reflections caused by headlight from the outside of the imaging plane. After the headlight image disappeared from the imaging plane, the auto gain function changes the setting into a high gain. Thus, before the change, the ghost image did not stand out since it was dark, but after the change, the ghost image becomes bright and is displayed. Hence, there is concern that the brightly displayed ghost image is erroneously recognized as headlight of another new vehicle, at the time when the gain is switched.